1. Technical Field
The present invention relates to a light source device, an image display device, and a monitor device.
2. Related Art
In recent years, coherent light sources have become fundamental also to image display devices, the optical communication field, the medical field, and the measurement field such as microscopes (see, e.g., JP-A-8-190111 (Document 1) and JP-A-5-13862 (Document 2)).
The high-frequency generator is provided with an LD, a bandpass filter, and a resonator having a nonlinear optical crystal. Thus, when the light with a fundamental wave emitted from the LD passes through the bandpass filter, the light in a wavelength band of ±0.5 nm around the fundamental wave is resonated. Therefore, since it can be prevented that the light is stabilized at a wavelength shifted out of a phase matching condition, it becomes possible to obtain high power light with harmonic component by making the light pass through the nonlinear optical crystal.
Further, the laser device described in the Document 2 is disposed having a laser medium held between a total reflection mirror and a partial reflection mirror. Further, a bandpass filter is disposed between the laser medium and the partial reflection mirror, thereby obtaining a laser beam with narrow spectrum. Thus, the laser beam emitted from the laser device becomes stabilized light.
However, the laser sources described in the Document land the Document 2 have a disadvantage of having coherency. According to this disadvantage, since an interference pattern appears on a projection surface on which the laser beam is projected as the speckle noise to degrade the image, some countermeasures to the speckle noise becomes necessary in order for displaying a high-resolution image.
As a measure for eliminating the speckle noise, there is proposed a device provided with a plurality of light emitting elements having central wavelengths slightly different in design from each other as an array (see, e.g., JP-T-2004-503923 (Document 3, the term “JP-T” as used herein means a published Japanese translation of a PCT patent application). In the technology described in the Document 3, the light emitting elements are provided as an array to obtain a wider spectrum band in comparison with the case of using a single light emitting element, thereby making it possible to reduce the speckle noise.
However, the device described in the Document 1 for eliminating the speckle noise is based on the premise of using the light source not requiring an external resonator structure, namely the light source directly outputting the laser beam. There is exerted the effect of suppressing the speckle noise in the case with the light source not requiring the external resonator structure, to be sure.
Here, in the case with a light source provided with an external resonator, the fundamental constituents are a light emitting element and a resonator mirror. Further, even in the case of using a plurality of light emitting elements, the resonator mirror for selecting a single wavelength is commonly used taking the cost and easiness of assembling into consideration. In the resonator mirror, the band of the wavelength to be selected needs to be narrowed in order for causing the laser oscillation. As a result, as described in the Document 3, even if variations are provided to the wavelengths of the light emitted from the arrayed light source, a single wavelength is selected by the wavelength selection element, the coherence of the entire light source including the wavelength selection element is not lowered.
Further, the resonator mirror in which the band of wavelength to be selected is narrowed and which reflects the light emitted from the light source is extremely expensive, thus increasing the total cost.